1. Field of the Invention
The present invention relates to a variable capacitor whose capacitance value can be controlled by voltage and a liquid crystal display device including the variable capacitor.
In this specification, a semiconductor device means all types of devices which can function by utilizing semiconductor characteristics, and a semiconductor circuit, a memory device, an imaging device, a display device, an electro-optical device, an electronic device, and the like are all semiconductor devices.
2. Description of the Related Art
As an element whose capacitance value can be controlled by voltage, a variable capacitor which is called a variable capacitance diode, a variable reactor, a varicap, and the like is known. As a variable capacitor, a pn-junction variable capacitor and an MIS (Metal-Insulator-Semiconductor) variable capacitor (also referred to as an MOS variable capacitor) are known.
A pn-junction variable capacitor utilizes a property that, when a reverse voltage is applied thereto, the thickness of a depletion layer which is generated at a pn junction interface varies depending on the level of the applied voltage, and variation in the capacitance value thereof can be adjusted by adjusting the impurity concentrations of p-type and n-type semiconductor layers.
In an MIS variable capacitor, an insulating layer and an electrode are formed over a semiconductor substrate, and the thickness of a depletion layer which is generated at a surface of the semiconductor substrate is controlled by the voltage applied to the electrode, whereby the capacitance value is varied. Although the capacitance ratio of the MIS variable capacitor cannot be made as large as that of the pn-junction variable capacitor, the MIS variable capacitor has an advantage that it can be formed at the same time as an MIS transistor and therefore can be highly integrated easily.
However, also in the MIS variable capacitor, the impurity concentration in the semiconductor substrate which is p-type or n-type needs to be controlled appropriately in order to deplete the semiconductor substrate. There is also a problem in that, although the capacitance value of the MIS (MOS) variable capacitor can be changed obviously depending on whether a positive bias voltage or a negative bias voltage is applied to the electrode in the case where a signal supplied to the electrode is a high-frequency signal with a frequency of 1 kHz or higher, the capacitance value cannot be changed obviously in the case where the frequency of the signal is lowered, and especially in the case of a low-frequency signal with a frequency of 100 Hz or lower or a direct-current signal, the capacitance value hardly changes regardless of whether a positive bias voltage or a negative bias voltage is applied to the electrode.
As an example of the MIS variable capacitor, a variable capacitor disclosed in Patent Document 1 can be given. The variable capacitor disclosed in Patent Document 1 has a structure in which a p-type silicon layer or an n-type silicon layer, and an insulating layer are provided between two electrodes having different areas and the p-type silicon layer or the n-type silicon layer is depleted by a voltage applied between the electrodes, whereby two different capacitance values are realized.